1. Field of the Invention
This invention relates to phase-type diffusing plates. In application to, for example, the finder of the photographic camera, it relates to phase-type focusing screens having prescribed diffusion characteristics so that a good finder image can be observed.
2. Description of the Related Art
The phase-type diffusing plate has a plurality of minute patterns formed in a surface of an optically transparent base plate to a certain height so that the ones of the light rays passing through the diffusing plate which emerge from the minute patterns differ in phase from the others. Thus, the diffusing plate supplies predetermined diffusion characteristics. A wide variety of forms of the phase-type diffusing plate have been proposed.
Of these, a bi-level form of the phase-type diffusing plate whose minute patterns give the entering light bundle a phase difference of zero and another phase difference which has a predetermined value .theta., totaling two phase differences, is proposed in, for example, Japanese Laid-Open Patent Applications Nos. Sho 55-70827 and Sho 62-269157.
Another form which gives the entering light bundle the phase difference of zero and phase differences of .theta..sub.1 and .theta..sub.2, totaling three phase differences, is proposed in U.S. patent application Ser. No. 482,812 filed on Feb. 21, 1990 where the phase-type diffusing plate is made up with three plane portions A, B and C of different levels, two of which each have patterns of round or oval shape. In the above-cited U.S. Patent Application, labeling these plane portions A, B and C in the order from the thinnest thickness beneath the pattern, the plane portion A corresponds to the base plate (reference surface), while the other plane portions B and C have different heights from each other. The plane portions A, B and C then impart into the traveling light rays traveling thereacross respective different phase differences. Thus, a phase-type diffusing plate having 3 levels of different phase differences is made up.
The bi-level form of the phase-type diffusing plate has a problem that, when used as the focusing screen in the photographic system, the finder image suffers coloring and failure of uniform brightness, being difficult to observe comfortably.
Another problem is that, when the telephoto lens, close-up lens, or like lens which is largely different in the position of the pupil from the standard lens is used in combination with a stop at a small aperture, a spot having the same color as that of the 0th order diffracted light arises at the center of the field of view.
Unlike this, in the 3-level phase-type diffusing plate proposed in the above-cited U.S. Patent Application, these problems have been solved, so a finder image of no color unevenness and, moreover, rich corner brightness at the full open aperture of the stop can be observed.